1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus including a magnetic recording medium having a magnetic recording layer formed in a predetermined convex-concave pattern on a substrate and thus having so-called servo areas and information data areas (a magnetic recording medium of a discrete type) and a magnetic head for detecting servo signals on the magnetic recording medium and recording and reproducing information data on and from the medium.
2. Description of the Related Art
Improvement in areal recording density of magnetic recording mediums such as hard disks has conventionally been achieved by techniques of both (1) improving the linear recording density and (2) improving the track density. In order to achieve further and higher densification in future, it is necessary to improve the areal recording density based on the foregoing both techniques.
With respect to improving the track density, there have been raised problems of processing limitation about magnetic heads, side-fringe or crosstalk caused by expansion of magnetic fields of magnetic heads, and so forth, and therefore, it can be said that the improvement in areal recording density by progressing the track-density increasing technique for magnetic heads, which is merely an extension of the conventional improvement technique, has reached the limit.
On the other hand, as a technique of improving the linear recording density, reduction in layer thickness and higher coercive forces have been achieved in conventional longitudinal magnetic mediums. However, in terms of further and higher densification of the mediums and stability of recording magnetization against thermal fluctuation, attention has been paid to perpendicular magnetic recording mediums.
Under these circumstances, as a technique of improving the areal recording density and supplementing the higher track densification of the magnetic heads, there have been proposed magnetic recording mediums of a discrete track disk type in which a recording layer is formed in a predetermined convex-concave pattern. For example, JP-A-H11-328662 discloses a magnetic recording medium in which predetermined convex and concave portions are formed on a substrate and a perpendicular magnetic layer in the form of a single layer is formed along the convex and concave portions.
A reduction in spacing is necessary for accomplishing an increase in recording density. However, there is a possibility that the convex-concave shape of the recording layer may impede the stable flying characteristic of a magnetic head and thus cause a problem of head crash or the like. From this point of view, JP-A-H10-222944 discloses a recording medium in which the convex-concave shape changes in a track width direction for the purpose of achieving the flying stability of a magnetic head.
Further, JP-A-2000-195042 proposes a discrete type magnetic recording medium in which concave portions in the convex-concave shape are filled with a nonmagnetic material or another material for ensuring the stability in flying characteristic of a magnetic head.
On the other hand, JP-A-H06-111502 discloses a technique that defines a relationship among the width of each of rectangular tracking servo burst patterns formed by a convex-concave structure on a longitudinal recording medium, the track pitch, and the read width of a reproducing head.
In general, on a magnetic recording medium used in a magnetic disk drive, servo areas necessary for a magnetic head to perform tracking are recorded by a servo track writer.
The servo area generally includes an ISG (Initial Signal Gain) portion, an SVAM (SerVo Address Mark) portion, a Gray code portion, a burst portion, and a pad portion which are in the form of various magnetic patterns for exhibiting predetermined functions, respectively.
The magnetic patterns of the burst portion are each recorded with a width equal to that of one track in a radial direction of the magnetic recording medium. On the other hand, the ISG portion, the SVAM portion, the Gray portion, and the pad portion are each recorded continuously in the disk radial direction over several tracks or entirely.
The recent increase in recording density of magnetic recording apparatuses has been remarkable and, following it, the sizes of recording bits recorded on magnetic recording mediums have been reduced. Consequently, a reduction in size of magnetic grains is also required for ensuring high S/N ratios. In this regard, it is said that, in connection with the longitudinal magnetic mediums which have been widely used, when a value of KuV/kT, i.e. a ratio between a magnetic grain magnetization energy KuV (Ku: magnetic anisotropy constant, V: magnetic grain volume) and an ambient temperature thermal energy kT (k: Boltzmann's constant, T: absolute temperature), becomes smaller than about 60 as a general standard, the so-called phenomenon of thermal fluctuation occurs wherein the magnetization fluctuates with certain probability due to disturbance of the thermal energy and decreases with the passage of time.
In view of this, in order to increase the magnetization energy of the magnetic grains, attention has been paid to the perpendicular magnetic recording mediums that can increase the thickness thereof even at high density.
The perpendicular magnetic recording medium becomes more stable in its magnetization and thus becomes stronger against the thermal fluctuation as the density increases, while, since a diamagnetic field serving to reduce the magnitude of the magnetization increases at low recording density, i.e. in an area where the bit length is large, the influence of the thermal fluctuation tends to be accelerated to reduce the recording magnetization.
Therefore, the area that is most affected by the thermal fluctuation is the servo area where servo signals are recorded at relatively low recording density.
Once recorded, the servo signal is normally not recorded again by the magnetic head, and therefore, it is susceptible to the influence of thermal fluctuation over the long term so that there may arise a problem that the servo signal is degraded due to a reduction in recording magnetization, resulting in reduction of the tracking servo signal quality.
In view of such a problem, JP-A-H11-25402 discloses a technique wherein, in recording magnetization of tracking servo signals in a perpendicular magnetic recording medium having no convex-concave structure, the bit lengths of the servo signals are set such that a maximum demagnetization field at the time of magnetization saturation in recording bits becomes smaller than a coercive force of a recording layer, and wherein relational expressions for the setting are derived.
In this proposed technique, the servo signals are continuously arranged while perpendicular magnetizations M of rectangular bits are alternately inverted as shown in figures of this publication, and therefore, a diamagnetic field Hd generated at a certain bit is reduced in its diamagnetic field due to magnetic fields H from the most adjacent bits. Particularly, the diamagnetic field theoretically becomes zero in a boundary between the bits and it is thus considered that the value approximate to the saturation magnetization is ensured in the vicinity of ideal magnetization transition.
However, as shown in JP-A-H06-111502, in case of the discrete track disk having the convex-concave structure, rectangular recording bits are normally arranged at intervals of one bit in a burst portion which is in the convex-concave shape of a magnetic recording layer corresponding to the servo patterns. Further, rectangular bits each elongate in the disk radial direction are arranged at intervals of one bit in an ISG portion, an SVAM portion, and a Gray code portion.
In the discrete track disk where the magnetic recording layer is processed into the convex-concave structure as described above, as is different from the system in which the inverted magnetizations are continuously recorded on the perpendicular magnetic medium as described in JP-A-H11-25402, convex portions, where servo signals are recorded, of the magnetic recording layer are completely isolated from each other so that there exist no such most adjacent bits that are inversely magnetized and serve to weaken a diamagnetic field. Further, those bits distanced from each other by one bit length are magnetized in the same direction so that the effect of reducing the diamagnetic field is hardly expected and they rather serve to increase the diamagnetic field.
Therefore, there is a demand for design guidelines for the magnetic recording layer convex portions for the servo signals that are formed completely isolated from each other.
On the other hand, when forming the convex-concave structure of a perpendicular magnetic recording layer (magnetic layer), it is possible to shorten the operation time and reduce the production cost as the depth of a concave portion is set smaller. Therefore, it can be said that the manner of forming a convex-concave magnetic layer without completely removing the magnetic layer in the depth direction and of filling concave portions with a nonmagnetic material is a desirable way in terms of simplifying the process and reducing the operation cost.
However, if the convex-concave shape of the magnetic layer is determined without taking a scheme into consideration, there may arise a problem that the servo signal output decreases in relation to the magnitude of the depth of the concave portion of the magnetic layer. Particularly, there is a possibility that various degradation modes may be caused by parameters of the convex-concave structure of the perpendicular magnetic recording layer (magnetic layer), the depth of the concave portion thereof, and further, magnetic properties, applied herein, of the perpendicular magnetic recording layer (magnetic layer) in the servo area. Therefore, with respect to servo tracking signals at the convex portions of the magnetic layer formed in the convex-concave pattern, it has been difficult to ensure a sufficient long-term reliability against thermal fluctuation.
The present invention has been made under these circumstances and has an object to provide a magnetic recording and reproducing apparatus including a perpendicular magnetic recording medium having servo tracking signals at convex portions of a magnetic layer formed in a convex-concave pattern, wherein the magnetic recording medium can suppress degradation of the servo signals caused by thermal fluctuation to thereby ensure a stable servo function over the long term.